After building orogens with Dewey and Bird (1970) and extending them during building with Platt (1986), Dewey (1988) explores the lifetime of orogens in their last breath: their extensional collapse. Simple considerations on how to form a normally-thick continental crust (30-40 km) from a thickened continental domain (50-60 km) indicate that erosion of the topography alone is far from being sufficient: it is too slow compared to the geological record, and there is no correlation between the volume of sediments and the volume of “lost” crustal and lithospheric material. Dewey (1988) brings an elegant answer to this issue with this must read paper, which proposes and describes five major tectonic phases of orogenic evolution, and their related tectonic processes (Fig. 1):
- Orogen building with HP “Franciscan” metamorphic gradients (Fig. 1b);
- Thermal incubation with the characteristic MP “Barrovian” metamorphic gradient with variable amount of crustal melting;
And the collapse is triggered when:
- Removal of the “thermal boundary conductive layer”, i.e., the lithospheric root of the orogen, enhancing HT-LP metamorphism with advanced mantle partial melting (Fig. 1c);
- Isostatic reequilibration reducing the orogen topography through extensional deformation (Fig. 1d, initial and transient stages);
- Stabilization of the continental domain by lithospheric cooling to return back to standard lithospheric temperatures and re-equilibrate the lithosphere-asthenosphere boundary (Fig. 1d, final stage).
Figure 1: The various thermal states of an orogenic lithosphere (modified by Petri 2014, from Platt and England 1993).
In this context, the collapse itself is caused by a combination of isostatically compensated uplift and subduction rollback. And of course this orogenic extension is also a potential explanation for preserving and exposing at the surface major structural features of orogens, such as high-pressure metamorphic rocks.
Orogens are therefore important zones for lithospheric extension, since they have a thick continental crustal, pre-existing lithospheric weaknesses, and low viscosity layers in their root. As a corollary, Dewey highlights that, as proposed by Wilson (1966), opening of Earth’s oceans may preferentially localises along former orogenic systems, probably as a consequence of the profound reworking of the lithosphere and the various ways “inheritance” can be generated: this may condition, together with pre-existing orogenic-inherited discontinuities, the subsequent rifting processes (e.g., Peron-Pinvidic and Osmundsen 2020).
Figure 2: Theoretical models for orogenic collapse (Rey et al., 2001).
Dewey’s paper changed the view we have on the last million years of the life of orogens, and is at the source of a series of studies from the late-80’s to 90’s on their collapse (Fig. 2; Rey et al., 2001), as e.g., on the variscan orogen of Western Europe (Ménard and Molnar, 1988; Rey, 1993; Burg et al., 1994). The topography of ancient orogens is crucial to understand their evolution: an orogenic crustal and lithospheric root should be present, and therefore the isostatically compensated topography should be regionally high in order to trigger collapse dynamics (Figs 1-2). The challenge lies in accurately determining paleo-elevation, which leads scientific debate, as for the case of the variscan orogen (a generalized high elevation as in orogenic plateaus, Becq-Giraudon et al., 1996; or generally lower elevation with local high topography, Franke, 2014; Roscher and Schneider, 2006); more constraints will probably come in the near future, notably by the development of isotopic proxies for paleoaltimetry (e.g., Dusséaux et al., 2019)
Written by Gianluca Frasca, Benoît Petri, Pan Luo, Silvia Crosetto, and David Fernández-Blanco with the Must-read team
References
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